6. SUMMARY

We conclude this review by again referring to
Figure 1. These results on
the space density of galaxies as a function of central surface brightness
require a basic adjustment in the way we think about galaxies. Much of
the current thought is implicitly one dimensional, with one parameter
(like luminosity or morphology) dominating the way problems are approached.
This is no longer sufficient. Surface brightness selection effects have
been severe. Our results now indicate that up to 50% of the general
galaxy population is in the form of galaxies with central surface
brightness below 22.0 mag arcsec-2. Moreover, the space
density remains flat out to the limits of the data, and the space
density of the lowest surface brightness disks (µ0 25.0 mag
arcsec-2, the limits of current data) is vastly higher than
would have been anticipated based on Freeman's Law.

LSB galaxies offer a new window onto galaxy evolution. Because of this,
the quest to find LSB galaxies continues. Over the next few years we hope
to extend our sensitivity by using wide field CCD surveys of the sky at
dark sites. These and new surveys by other groups should extend the
current data by two magnitudes down to µ0 = 27.0
mag arcsec-2. The major
goals of these new surveys are to determine if the space density of
galaxies as a function of µ0 continues to remain
flat over a factor of 100 in µ0, and to detect
what we so far have failed to detect
in large numbers - the red LSB population that must result from the
faded remnants of galaxies that no longer can form stars. In this regard,
the Sloan Digital Sky Survey will be extremely helpful if its automatic
image recognition system can reliably detect LSB galaxies. At levels
well below the sky brightness, however, LSB galaxies are often defined by
disconnected regions of pixel intensity. While the human eye is
rather good at finding these, its not clear if that particular
algorithm can be reproduced in a machine.

Despite the power of CCD surveys and automated detection algorithms,
we are inevitably brought back to the starting point of this review.
Messier cataloged galaxies long before their extragalactic nature was
understood, and it is ironic that his catalog was a reject list of
stationary objects for comet-hunters to avoid. We now know a lot more
about about galaxies, but the selection biases that have operated for
200 years have not been fully overcome. It would be unwise to presume
that we have yet revealed the true population of galaxies.

We leave the reader with a small list of things that we hoped they
have learned from this review article:

(1) LSB galaxies exist. Lurking beneath the brightness of the night
sky are real galaxies with evolutionary histories substantially
different from the processes that produced the Hubble Sequence of
spirals. In particular, LSBs evolve at a significantly slower rate
and may well experience star formation outside of the molecular
cloud environment.

(2) Surface brightness selection effects have been severe. A proper
accounting of them has increased the local number density of galaxies
and steepened the faint end slope of the galaxy luminosity function.
Despite this progress, these selection effects still exist and thus
we do not yet have a representative, volume limited sample of nearby
galaxies.

(3) LSB galaxies span the entire galactic mass range. They are not
exclusively low mass galaxies but include the most massive disk
galaxies discovered to date (e.g., Malin 1). LSB disks likely are the
manifestation of 1-2
isolated peaks in the initial density
fluctuation spectrum. These lower density peaks have longer collapse
times and trace the mass distribution in a relatively unbiased way.

(4) LSB galaxies are embedded in dark matter halos which are of lower
density and more extended than HSB galaxy halos. In this sense, disk
galaxy surface mass density and subsequent evolution may be predetermined
by the form of the dark matter halo. Surface mass density appears to be
the single biggest driver of disk galaxy evolution.

ACKNOWLEDGMENTS

A number of people have helped to support this project over the
last decade. We gratefully acknowledge Mike Disney for making
us think, David Malin for his wizardry
and patience with us, Jim Schombert for being there, Steve Strom
for pointing the way, Jay Gallagher for telling one of us (G.D.B.) to
work on something ``hard'', Mark Cornell for assistance with
the Texas observations, and Allan Sandage for originally showcasing
the smudge galaxies. We gratefully acknowledge support from the NSF under
grant AST-9005115 and AST-9003158 without which, this project could
never have been sustained.